Magnesium alloys have been used for many years in applications where the material of construction is required to exhibit a high strength to weight ratio. Typically a component made from a magnesium alloy could be expected to have a weight about 70% of an aluminium (Al) alloy component of similar volume. The aerospace industry has accordingly been a significant user of magnesium alloys and magnesium alloys are used for many components in modern defence aircraft and spacecraft. However, one limitation preventing wider use of magnesium alloys is that, when compared to aluminium alloys, they typically have poorer resistance to creep at elevated temperatures.
With the increasing needs to control international fuel consumption and reduce harmful emissions into the atmosphere, automobile manufacturers are being pressured into developing more fuel efficient vehicles. Reducing the overall weight of the vehicles is a key to achieving this goal. A major contributor to the weight of any vehicle is the engine itself, and the most significant component of the engine is the block, which makes up 20–25% of the total engine weight. In the past significant weight savings were made by introducing an aluminium alloy block to replace the traditional grey iron block, and further reductions of the order of 40% could be achieved if a magnesium alloy that could withstand the temperatures and stresses generated during engine operation was used. However, the development of such an alloy, which combines the desired elevated temperature mechanical properties with a cost effective production process, is necessary before a viable magnesium engine block manufacturing line could be considered. In recent years, the search for an elevated temperature magnesium alloy has focused primarily on the high pressure die casting (HPDC) processing route and several alloys have been developed. HPDC was considered to be the best option for achieving the high productivity rates required to counteract the probable high cost of the base magnesium alloy. However, HPDC is not necessarily the best process for the manufacture of an engine block and, in reality, the majority of blocks are still precision cast by gravity or low pressure sand casting.
There are two major classes of magnesium sand casting alloys.
(A) Alloys based on the magnesium-aluminium binary system, often with small additions of zinc (Zn) for improved strength and castability. These alloys have adequate room temperature mechanical properties, but do not perform well at elevated temperatures and are inappropriate at temperatures in excess of 150° C. These alloys do not contain expensive alloying elements and are widely used in areas where high temperature strength is not a requirement.
(B) Alloys able to be grain refined by the addition of zirconium (Zr). The major alloying elements in this group are zinc, yttrium (Y), silver (Ag), thorium (Th), and the rare earth (RE) elements such as neodymium (Nd). Throughout this specification the expression “rare earth” is to be understood to mean any element or combination of elements with atomic numbers 57 to 71, ie. lanthanum (La) to lutetium (Lu). With the right choice of alloying additions, alloys in this group can have excellent room and elevated temperature mechanical properties. However, with the exception of zinc, the alloying additions within this group, including the grain refiner, are expensive with the result that the alloys are generally restricted to aeronautical applications.
The magnesium alloy ML10, developed in the USSR, has been used for many years for cast parts intended for use in aircraft at temperatures up to 250° C. ML10 is a high strength magnesium alloy developed on the basis of the Mg—Nd—Zn—Zr system. ML19 alloy additionally contains yttrium.
A paper by Mukhina et al entitled “Investigation of the Microstructure and Properties of Castable Neodymium and Yttrium-Bearing Magnesium Alloys at Elevated Temperatures” published in “Science and Heat Treatment” Vol 39, 1997, indicated typical compositions (% by weight) of ML10 and ML19 alloys are:
ML10ML19Nd2.2–2.81.6–2.3YNil1.4–2.2Zr0.4–1.00.4–1.0Zn0.1–0.70.1–0.6MgBalanceBalancewith impurity levels of:
Fe<0.01Si<0.03Cu<0.03Ni<0.005Al<0.02Be<0.01
Alternatives which have been developed are alloys known to those in the art as QE22 (an Mg—Ag—Nd—Zr system alloy) and EH21 (an Mg—Nd—Zr—Th system alloy). However, these alternatives are expensive to manufacture as they contain significant quantities of silver and thorium respectively.
Heat resistant grain refined magnesium alloys can be strengthened by a T6 heat treatment which comprises an elevated temperature solution treatment, followed by quenching, followed by an artificial aging at an elevated temperature. In heating before quenching the excess phases pass into solid solution. In the aging process refractory phases, in the form of finely dispersed submicroscopic particles, are segregated and these create microheterogeneities inside the grains of the solid solution, blocking diffusion and shear processes at elevated temperatures. This improves the mechanical properties, namely the ultimate long term strength and the creep resistance of the alloys at high temperature.
To date, a sand casting magnesium alloy having desired elevated temperature (eg 150–200° C.) properties at a reasonable cost has been unavailable. At least preferred embodiments of the present invention relate to such an alloy and the present invention is particularly, but not exclusively, directed to application with precision casting operations.